An array antenna includes an array of antenna elements for transmission or reception of electromagnetic signals. The antenna elements are fed with one or more signals whose amplitudes and phases are determined to form a beam, i.e., an array antenna signal in a specified direction. Typically, the relative amplitudes of each element signal are fixed by attenuators set at appropriate levels to shape the beam, while phase shifters connected to the elements are adjusted for changing the phases of the signals to steer the beam.
To precisely control the beam, the actual phase response of each phase shifter must be known. However, phase response of a phase shifter is subject to unavoidable errors and variations due to manufacturing discrepancies and to various changes occurring as a function of time and temperature. Thus, calibration is required to provide phase correction for each phase shifter. The phase calibration data can be stored and used during steering operations to correct phase response errors.
The amplitudes of the signals fed to the elements are adjusted with attenuators connected to the elements. The attenuators are also subject to errors and variations. Thus, calibration is required to provide attenuator calibration data for each attenuator. The attenuator calibration data can be stored and used during steering operations to correct attenuator response errors.
Previous methods of phased array calibration have relied on scanning each element of the array through all of its phase values relative to the other elements and measuring the power of the array antenna signal at each phase value. The measured phase value corresponding to maximum power is compared to the ideal phase value. The ideal phase value is the phase value corresponding to maximum power when there are no phase errors or variations. Thus, the difference between the measured phase value corresponding to maximum power and the ideal phase value is the phase error, or phase offset, for that element.
This procedure is repeated at least once for each element of the array. After the phase offsets for each element have been determined, the phases of the element signals are changed by their respective phase offsets to effect the calibration. Consequently, the errors are, at least currently, taken into account.
A problem with scanning each element through all of its phase values is that this requires a large number of measurements. For instance, phase values fall within the range of 0.degree. to 360.degree.. Thus, if the phase settings for each element were quantized in increments of 1.degree., then three hundred and sixty phase values must be scanned. If the array has a large number of elements, for example, one hundred, then at least three thousand six hundred measurements must be made for calibration of the array, and iteration may be required to improve accuracy. Scanning each element through all of its phase values is suboptimal in a noisy environment and has the disadvantage of potentially large interruptions to service.
Accordingly, a need has developed for a quicker and more efficient method which requires fewer measurements for calibrating an array antenna.